The present invention relates to an automotive vehicle brake system having a vacuum brake power booster which is interposed between brake pedal and master brake cylinder and which comprises at least two working chambers isolated from one another by a diaphragm plate, one chamber communicating with the vacuum source, while the other one is ventilated by means of a control valve actuatable by the brake pedal for generating a booster force that is proportional to the brake pedal force.
A brake system of this type is disclosed in German patent application No. P 38 14 222.8 (corresponding to the copending U.S. patent application of W. Wagner et al filed on Apr. 12, 1989 as Ser. No. 07/336,948 in which wheel brake cylinders are connected by way of brake lines to the master brake cylinder. In this system, sensors are allocated to the wheels to be braked which sense the rotational behavior of the wheels in order to determine wheel-lock, and the output signals of which sensors are delivered to a central control electronics. The control signals of the central control serve to control electromagnetically actuatable pressure-fluid inlet and outlet valves inserted into the brake lines for slip control. A second diaphragm plate is provided in the vacuum brake power booster to which a controllable pressure difference is applied and which isolates two additional pneumatic working chambers from each other and which, during slip control, is movable in a direction opposite to the actuating direction of the master brake cylinder under the effect of two compensating cylinders allocated to the master-brake-cylinder pressure chambers.
For generating in the slip control case a holdback force counteracting the brake pedal force, according to the noted German application, there is provision for a hydraulic chamber which is in operative connection with a force-output member of the vacuum brake power booster. The connection of the chamber with an unpressurized pressure-fluid supply reservoir is closable by way of a seat valve that preferably is actuatable electromagnetically. Relief pistons are slidably guided in each compensating cylinder, which confine relief chambers connectable to the master-brake-cylinder pressure chambers, and abut on the second diaphragm plate. The connections between the master-brake-cylinder pressure chambers and the relief chambers are closable by means of compensating valves which, preferably, are designed as solenoid valves closed in their de-energized state.
A significant disadvantage of these systems is the comparatively large number of valves which results in a potential source of trouble, and these systems are complicated and hence costly in construction. Therefore, it is an object of the present invention to improve upon such slip-controlled automotive vehicle brake systems such that the number of valves is reduced, while the system construction is simplified.